1. Field of the Invention
The present invention relates to logging of subterranean formations for determination of density using gamma rays. Particularly, this invention relates to determination of formation density while drilling a borehole traversing the earth formation.
2. Setting of the Invention
Wireline gamma ray density probes are devices incorporating a gamma ray source and a gamma ray detector, shielded from each other to prevent counting of radiation emitted directly from the source. During operation of the probe, gamma rays (or photons) emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. In photoelectric absorption and pair production phenomena, the particular photons involved in the interacting are removed from the gamma ray beam.
In the Compton scattering process, the involved photon only loses some of its energy while changing its original direction of travel, the loss being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered toward the detector. Many scattered rays do not reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process of the pair production process. The scattered photons that reach the detector and interact with it, are counted by the electronic equipment associated with the detector.
The major difficulties encountered in conventional gamma ray density measurements include definition of the sample size, limited effective depth and sampling, disturbing effects of undesired, interfering materials located between the density probe and the sample and the requirement that the probe is positioned against the borehole wall. The chemical composition of the sample also affects the reading of conventional gamma ray density probes. This is complicated further when the density measurement tool is made part of a drilling string and operated during drilling operations. There are no known density probes useful in measurement while drilling apparatus.
Applicant's application filed Mar. 11, 1983, now U.S. Pat. No. 4,596,926, discloses and claims an apparatus and method for determining formation density which utilizes three gamma ray sources in a common plane and located symmetrically about the axis of the sub with a set of three near detectors and a set of three far detectors also in common planes, each detector being in axial alignment with one of the gamma ray sources. The counting rates received by the detectors as the drill string rotates are used to provide an indication of average density of the formation sample.
One prior art wireline density probe disclosed in U.S. Pat. No. 3,202,822 incorporates two gamma ray detectors, one collimated gamma ray source and ratio building electronic circuits, and is useful as long as the interfering materials, located between the detectors of the probe and the formation sample, are identical in thickness and chemical composition along the trajectories of emitted and received gamma ray. Non-uniformities in the wall of the borehole will interfere with the proper operation of the probe. Such non-uniformities can be caused by crooked holes, by cave-ins, and by varying thicknesses of the mudcake on the wall of the hole.
The prior art also includes U.S. Pat. No. 3,846,631 which discloses a wireline density probe that functions regardless of the thickness and the chemical composition of materials that are located between the density probe and the sample. The method comprises passing of two gamma ray beams from two intermittently operated gamma ray sources into the sample, receiving the radiation backscattered from each of the two sources by two separate detectors, and building ratios of products of the four separate counting rates in such a manner that the numerical result is an indication of the density of the sample.
The critical dimension of the two-detector probe is the spacing between the detectors. If the interferring materials are non-uniform over distances comparable to the spacing of the two detectors, the measured density will be erroneous.
Neither of the wireline probes described above is disclosed as being useful for measurement when drilling and incorporation into a rotating drill string.